Connected: An Internet Encyclopedia
RFC 792 - Destination Unreachable Message

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### Destination Unreachable Message

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |     Code      |          Checksum             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             unused                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Internet Header + 64 bits of Original Data Datagram      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       IP Fields:

       Destination Address

          The source network and address from the original datagram's data.

       ICMP Fields:

       Type

          3

       Code

          0 = net unreachable;

          1 = host unreachable;

          2 = protocol unreachable;

          3 = port unreachable;

          4 = fragmentation needed and DF set;

          5 = source route failed.

       Checksum

          The checksum is the 16-bit ones's complement of the one's
          complement sum of the ICMP message starting with the ICMP Type.
          For computing the checksum , the checksum field should be zero.
          This checksum may be replaced in the future.

       Internet Header + 64 bits of Data Datagram

          The internet header plus the first 64 bits of the original
          datagram's data.  This data is used by the host to match the
          message to the appropriate process.  If a higher level protocol
          uses port numbers, they are assumed to be in the first 64 data
          bits of the original datagram's data.

       Description

          If, according to the information in the gateway's routing tables,
          the network specified in the internet destination field of a
          datagram is unreachable, e.g., the distance to the network is
          infinity, the gateway may send a destination unreachable message
          to the internet source host of the datagram.  In addition, in some
          networks, the gateway may be able to determine if the internet
          destination host is unreachable.  Gateways in these networks may
          send destination unreachable messages to the source host when the
          destination host is unreachable.

          If, in the destination host, the IP module cannot deliver the
          datagram  because the indicated protocol module or process port is
          not active, the destination host may send a destination
          unreachable message to the source host.

          Another case is when a datagram must be fragmented to be forwarded
          by a gateway yet the Don't Fragment flag is on.  In this case the
          gateway must discard the datagram and may return a destination
          unreachable message.

          Codes 0, 1, 4, and 5 may be received from a gateway.  Codes 2 and
          3 may be received from a host.

Next: Time Exceeded Message

Connected: An Internet Encyclopedia
RFC 792 - Destination Unreachable Message